1. Field of the Invention
The present invention relates to a conductive bonding agent and a conductive connecting structure.
2. Description of the Related Art
Various known methods are employed to electrically connect connecting terminals, which are spaced apart from each other, together. Soldering is one of the most commonly used methods for such a work. In order to satisfy a recent demand for a decrease in pitch of connecting terminals, the soldering techniques have been greatly improved. With the most advanced equipment and the greatest possible care, soldering can be applied to connection terminals at a pitch of about 200 .mu.m. However, the drawbacks of this method are that connecting terminals must have a wettability for soldering and hence must be made of at least conductive metal films, and that heat-resistant insulating boards need to be used because of high-temperature bonding. These conditions raise the costs of materials.
A method using a conductive adhesive is known as a method of connecting the connecting terminals formed on an inexpensive resin board to an electronic part. In this method, however, a conductive adhesive must be accurately positioned and put on each connecting terminal. Hence, this method is very inefficient. In addition, since intervened portions between the connecting terminals are not coated with adhesive, i.e., impossible to be bonded, only the connecting terminals are bonded with extremely low bonding strength, thereby reinforcing means for increasing the bonding strength is required. For this reason, the bonding portions occupy a large volume. In addition to these drawbacks, when the pitch of connecting terminals becomes 200 to 300 .mu.m or less, the frequency of short circuit and conduction failure is increased due to positioning with poor precision accompanying connecting operations.
As another conventional technique, a method using an anisotropic conductive adhesive is available, which is relatively close to the principle of the present invention, in comparison with the above-described method. An anisotropic conductive adhesive is obtained by dispersing fine conductive particles in an insulating adhesive. When the connecting terminals of a board are to be connected to the connecting terminals of an electronic part by using the anisotropic conductive adhesive, the adhesive is coated not only on the connecting terminals formed on the board and the electronic part, but also on intervened portions of the board and the electronic part between the connecting terminals. When the connecting terminals of the board and those of the electronic part are bonded together with the anisotropic conductive adhesive with heat and pressure, the respective fine conductive particles and the insulating adhesive located at the connecting terminals partially flows to intervened portions between the connecting terminals. The connecting terminals of the board and the electronic part are brought into direct contact with the fine conductive particles. In this case, if the respective fine conductive particles even after they flow to intervening portions between the connecting terminals, are located to be sufficiently separated respectively, so as not to make contact with each other, the connecting terminals formed on the board and the electronic part are free from short circuits. That is, an anisotropic conductive adhesive is an adhesive which has an electric conductivity in the direction of thickness and insulating properties in the planar direction, i.e., has a directional electric conductivity. U.S. Pat. No. 4,113,981 and GB 2068645A disclose such anisotropic conductive adhesives, and may be referred to for easy understanding thereof.
Since this anisotropic conductive adhesive contains a material, as an insulating adhesive, which melts at a relatively low temperature of 100.degree. to 200.degree. C., it can be applied to a resin board. In addition, when the anisotropic conductive adhesive is to be coated on the connecting terminals of a board, no accurate positioning work is required, and hence an effective operation can be performed. Moreover, since the adhesive is also interposed between the connecting terminals, the bonding strength can be increased.
As described above, it is absolutely essential for an anisotropic conductive adhesive to exhibit conductivity in the direction of thickness and insulating properties in the planar direction. In order to obtain conductivity in the direction of thickness, at least (theoretically) one fine conductive particle must be interposed between each connecting terminal of a board and a corresponding connecting terminal of an electronic part. In order to obtain insulating properties in the planar direction, every fine conductive particle is ideally separated from adjacent particles by insulating adhesive filled up thereof, so as not to be electrically conductive to each other. A part of adjacent fine conductive particles is allowed to make contact with each other, provided all of them are insulated from the adjacent connecting terminals. However, there is no guarantee that every conductive particle is insulated from the adjacent connecting terminals. Therefore, the anisotropic conductive adhesive needs to have a structure allowing no conduction between any adjacent fine conductive particles.
In an anisotropic conductive adhesive, however, the arrangement of fine conductive particles dispersed in an insulating adhesive is determined simply by the manner of stirring. For this reason, the distribution of the fine conductive particles inevitably becomes nonuniform, having dense and sparse portions. Therefore, such an anisotropic conductive adhesive can be used, provided that fine conductive particles in a dense portion are not conductive each other and at least one fine to particle is positioned to one connecting terminal in a sparse portion.
In proportion to a decrease in pitch of connecting terminals, and hence a decrease in width of each connecting terminal, the above-described conditions become more difficult to satisfy. The number of fine conductive particles located on one connecting terminal is decreased with a decrease in width of the connecting terminal. If the ratio of fine conductive particles mixed in the insulating adhesive is increased in order to increase the number of fine conductive particles located on each connecting terminal, the density of fine conductive particles in each dense portion is further increased. As is apparent, if intervened portions between connecting terminals, in the dense portions, are filled with fine conductive particles, the circuit is shorted.
Because of the above-described structure and function, the connecting method using an anisotropic conductive adhesive is theoretically applicable to the connection of connecting terminals at a pitch several times the diameter of a fine conductive particle. In practice, however, the actual application of this method is far from the theoretical assumption. If, for example, fine conductive metal particles each having a diameter of about 10 to 20 .mu.m are used, the pitch of connecting terminals can be only reduced to 200 to 300 .mu.m. In this case, the mixing ratio of the fine metal particles is about 15% in weight ratio. As long as this method is employed, practically as well as theoretically, an anisotropic conductive adhesive cannot be applied to the connection of connecting terminals having a pitch smaller than the diameter of a fine conductive particle.